Initial studies of a wide variety of retrovirus induced tumors suggesting a close ontogenic relationship between the B cell and myeloid lineages have been complimented with experiments involving transfer of normal cells into immunodeficient or irradiated mice. We have found that, contrary to current views of hematopoietic cell development, the B cell and the myeloid lineages may share a common progenitor cell and that this cell may be identified by the expression of the putatively monocyte/macrophage restricted antigen Mac-l. Mac-l was also found to be coexpressed with Thy-l on a pluripotent stem cell. We have studied an antigen recognized by our new monoclonal antibody, LIP-6 which is expressed by Ly-l B cells in the peritoneum and on peritoneal macrophages but not on peripheral B cells, T cells or granulocytes. This antigen may help to further elucidate the relationship between the B and myeloid lineages. The effects of the BM-5 virus on the bone marrow was found to be most profound on B lymphopoiesis, initially stimulating production of B cells followed by a decrease in the ability of stromal cells to support the growth of normal cells. Studies of immunodeficient RIIIS/J mice have suggested, contrary to other reports, -that within the Ly-l B cell lineage, the related Ly-l+ and the Ly-1 B cells may differ functionally.